The present invention relates to the field of communications in general and more particularly, to determining the position of a mobile terminal device.
Wireless communication systems (networks) are commonly employed to provide voice and data communications to subscribers. For example, analog cellular radiotelephone systems, such as those designated AMPS, ETACS, NMT-450, and NMT-900, have long been deployed successfully throughout the world. Digital cellular radiotelephone systems such as those conforming to the North American standard IS-54 and the European standard GSM have been in service since the early 1990's. More recently, a wide variety of wireless digital services broadly labeled as PCS (Personal Communications Services) have been introduced, including advanced digital cellular systems conforming to standards such as IS-136 and IS-95, lower-power systems such as DECT (Digital Enhanced Cordless Telephone) and data communications services such as CDPD (Cellular Digital Packet Data). These and other systems are described in The Mobile Communications Handbook, edited by Gibson and published by CRC Press (1996).
In addition to such regulated wide area cellular networks, localized short range transmission protocols and wireless local area networks (WLAN) are also known. For example, the wireless coupling between the handheld electronic device and the video signal generator can be provided, for example, using the Bluetooth protocol for short range transmission (typically up to a maximum distance of 100 meters). The Bluetooth protocol is discussed, for example, by Sailesh Rathi in the reference entitled “Blue Tooth Protocol Architecture” from Dedicated Systems Magazine, 2000 Q4, pages 28-33, the disclosure of which is hereby incorporated herein in its entirety by reference. Wireless local area networks providing access points to the Internet are becoming more common at work places, coffee shops and the like.
It is desirable, and in certain places mandated by law, that mobile telecommunication network providers be able to determine an approximate geographical location of a mobile terminal (MT), such as, for example, an actively communicating cellular telephone.
A variety of MT location techniques have been proposed. These location techniques include uplink signal location, downlink signal location, Global Positioning System (GPS) based approaches, assisted GPS approaches combining communication signals and GPS signals and approaches based on digital television signals. For “uplink signal” location techniques, the mobile telecommunications network is typically configured to determine where the MT is located based on ranging measurements associated with one or more uplink signals. These uplink signals are transmitted by the MT and received by a number of receivers having known locations, such as, for example, cellular telephone base stations (BSs). For the “downlink signal” location techniques, the mobile telecommunications network is typically configured to determine where the MT is located based on ranging measurements associated with the reception, by the MT, of downlink signals from a number of transmitters having known locations.
FIG. 1 illustrates a conventional terrestrial wide area mobile (wireless) communications network 20 that may implement any one of a variety of known wireless communications standards including uplink and downlink signals. The wireless network may include one or more wireless mobile terminals 22 that communicate with a plurality of cells 24 served by base stations 26 and a mobile telephone switching office (MTSO) 28. Although only three cells 24 are shown in FIG. 1, a typical cellular radiotelephone network may comprise hundreds of cells, and may include more than one MTSO 28 and may serve thousands of wireless mobile stations 22.
The cells 24 generally serve as nodes in the network 20, from which links are established between wireless mobile terminals 22 and a MTSO 28, by way of the base stations 26 servicing the cells 24. Each cell 24 will have allocated to it one or more dedicated control channels and one or more traffic channels. The control channel is a dedicated channel that may be used for downlink transmission (network to mobile) of cell identification and paging information. The traffic channels carry the voice and data information. Through the network 20, a duplex (downlink and uplink) radio communication link 30 may be effected between two wireless mobile stations 22 or between a wireless mobile station 22 and a landline telephone user 32 via a public switched telephone network (PSTN) 34. The function of the base station 26 is commonly to handle the radio communications between the cell 24 and the wireless mobile station 22. In this capacity, the base station 26 functions chiefly as a relay station for data and voice signals. It is also know to provide wide area wireless communications networks in which the functions provided by the base stations are provided by satellites, having associated coverage areas, rather than terrestrial base stations.
The GPS location approach generally uses location services not associated with either the uplink or downlink signals used in the mobile telecommunications network. In a typically GPS application, the GPS receivers collect and analyze ranging measurements from signals transmitted by GPS satellites having known locations.
As illustrated in FIG. 2, GPS is a space-based triangulation system using satellites 42 and GPS control computers 48 to measure positions anywhere on the earth. GPS was first developed by the United States Department of Defense as a navigational system. The advantages of this navigational system over land-based systems are that it is not limited in its coverage, it provides continuous 24-hour coverage, which may be highly accurate regardless of weather conditions. In operation, a constellation of 24 satellites 42 orbiting the earth continually emit a GPS radio signal 44. A GPS receiver 46, e.g., a hand-held radio receiver with a GPS processor, receives the radio signals from the visible satellites and measures the time that the radio signal takes to travel from the GPS satellites to the GPS receiver antenna. By multiplying the travel time by the speed of light, the GPS receiver can calculate a range for each satellite in view. Ephemeris information provided in the satellite radio signal typically describes the satellite's orbit and velocity, thereby generally enabling the GPS processor to calculate the position of the GPS receiver 46 through a process of triangulation. It is known to include a GPS receiver 46 in a mobile station 22 to provide position location functionality to the mobile station 22.
The startup of a GPS receiver typically requires the acquisition of a set of navigational parameters from the navigational data signals of four or more GPS satellites. This process of initializing a GPS receiver may often take several minutes. The duration of the GPS positioning process is directly dependent upon how much information a GPS receiver has initially. Some GPS receivers are programmed with almanac data, which coarsely describes the expected satellite positions for up to one year ahead. However, if the GPS receiver does not have some knowledge of its own approximate location, then the GPS receiver may not be able to find or acquire signals from the visible satellites quickly enough, and, therefore, cannot calculate its position quickly. Furthermore, it should be noted that a higher signal strength is typically needed for capturing the navigation data at start-up than is needed for continued monitoring of an already-acquired signal. It should also be noted that the process of monitoring the GPS signal may be significantly affected by environmental factors. Thus, a GPS signal which may be easily acquired in the open typically becomes harder to acquire when a receiver is under foliage, in a vehicle or in a building.
It is known to provide mobile terminals with accurate location assistance information, e.g., local time and position estimates, satellite ephemeris and clock information, and visible satellite list (which generally varies with the location of the mobile station). The use of such assistance data can permit a GPS receiver that is integrated with or connected to a mobile terminal to expedite the completion of its start-up procedures.
Taylor et al., U.S. Pat. No. 4,445,118, discusses the concept of aiding or assisting GPS receivers. The method described uses a single transmitter, such as a geosynchronous satellite, to provide a single assistance message for a wide geographical area. The assistance message data includes a list of GPS satellites in view, the respective satellite positions, and predicted Doppler shifts on the satellite signals. This structure of this message permits the position computation function (PCF) to be done in the user receiver.
Krasner, U.S. Pat. No. 5,663,734, describes another GPS receiver approach. This patent is mainly related to the receiver architecture, but discusses how the receiver performance can be improved by assistance. The patent mentions “data representative of ephemeris” and expected Doppler shifts as possible contents of the assistance message.
Lau, U.S. Pat. No. 5,418,538, describes a system and method for aiding a remote GPS/GLONASS receiver by broadcasting “differential” information from a like receiver in a “reference station.” The reference station broadcasts a visible satellite list and also the associated ephemeris, in one embodiment. The advantages to the remote receiver may be three-fold: reduced memory requirements, lower-cost frequency reference, and faster acquisition. The discussion describes the benefit of being able to estimate and remove the Doppler shift due to the receiver clock inaccuracy after acquiring the first satellite.
Eshenbach, U.S. Pat. No. 5,663,735, describes a method whereby a GPS receiver derives an accurate absolute time reference from a radio signal. Optionally, the receiver also derives from the radio signal a frequency reference that is more accurate than the inexpensive crystal oscillator contained in the receiver. The GPS receiver performs the position calculation, and therefore must have the absolute time as well as the ephemeris and clock corrections for the GPS satellites.
Another assisted-GPS standard for wide area wireless networks, in particular GSM-based networks, is described in specification numbers 3GPP TS 04.31, 04.35 and 3GPP TS 03.71. This standard is based on placing reference GPS receivers at various nodes in the network, capturing the ephemeris information from these receivers, then providing this information along with a list of visible satellites and/or other information to all handset-based GPS receivers via messages on GSM downlink bearers. The benefit of this approach is that it allows the handset-based GPS receiver to be fully functional, i.e., it contains the position computation function and also can operate in continuous navigation mode. However, to obtain such data from the GSM-based network, the GSM network provider must make the data available. Furthermore, it is expected that the providers may require a subscription or other relationship with a mobile terminal requesting the information before it is provided. Even if a user of a mobile terminal has a subscription with a local provider, that subscription may not entitle the user to access the assistance data in other networks when traveling.